(1) Field of the Invention
The present invention relates to liquid crystal display devices. This invention more specifically relates to novel backlight technology for liquid crystal display devices.
(2) Background Information
The importance of electronic displays has increased substantially in recent years with the proliferation of computer technology. Many industry analysts foresee the day when wall-size flat panel displays (FPDs) will be common features in offices, homes and movie theaters. Towards that end, worldwide sales of FPDs, which are already substantial, are expected to increase by a factor of four between calendar years 2001 and 2006 to approximately $55 billion.
Currently, liquid crystal displays (LCDs) are the FPD technology of choice for most applications. Further, LCDs are considered to be the most promising technology for meeting the demands of future FPD applications. However, there remain several technical barriers that limit the development and use of LCDs. One significant technical barrier is the low light efficiency of the conventional LCD. The light efficiency of a typical, conventional LCD panel is only about 3–4%. As a result, the typical LCD has a low display brightness, high energy consumption, and weighs more than desired for most applications. These limitations are particularly noteworthy in laptop computers, which is a major application for LCDs. Three important factors that reduce LCD light efficiency are:
Color filter loss: Most conventional LCD panels utilize white light backlight units. Red (R), green (G), and blue (B) color filters are utilized to achieve full-color image displays. A schematic of the basic structure of a conventional LCD panel 50 is shown in FIG. 1. A color filter 3 is shown adjacent to the backlight 1 side of a liquid crystal cell 4. (Alternatively, color filter 3 may also be positioned on the other side of the liquid crystal cell 4, e.g., adjacent the clear substrate 5 on the user-side of the cell 4.) In this typical example, color filter 3 is of the absorptive type, i.e. it transmits one of the primary colors but absorbs the other two. The theoretical maximum transmittance of this type of color filter 3 is about 33%. In practice a typical absorptive type color filter 3 has a transmittance of about 30%. Therefore, in a conventional LCD, about 70% of the incident light energy is absorbed in the color filter 3.
Polarizer loss: A linear polarizer 2 used in LCD panel 50 is also of the absorptive type. Light of the desired polarization orientation is transmitted, while that which is orthogonal to the desired polarization direction is absorbed. The theoretical transmittance of an absorptive type linear polarizer is 50% for incident light with a random polarization state. In practice, the transmittance of a linear polarizer is approximately 45%. Therefore, approximately 55% of the light incident on the linear polarizer 2 is absorbed.
Backlight unit loss. Conventional backlight units in LCD devices are typically composed of edge light lamps 1A and light guiding pipes 1B, as shown in FIG. 1. The purpose of the light pipes 1B is to guide light from the edge lamps 1A, which are located at the edge of the backlight units 1, towards the center of the LCD panel. Typically, about 44% of the light produced by the edge lamps 1A is lost in the light pipes 1B. Further, in order to achieve a uniform backlight distribution, a highly complicated structure of diffusing elements is necessary. Further still, the edge lamps 1A require a high voltage to produce an adequate supply of light. As a result the power consumption of LCD panels is relatively high.
The total loss from the above-mentioned factors is about 92%. Therefore, elimination of these losses would improve the light efficiency of the conventional LCD by greater than a factor of 11. This would likely result in a substantially brighter LCD panel with much lower power requirements (resulting in longer battery life for most portable electronic applications).
Several approaches have been attempted to eliminate the above-mentioned losses. A self-emitting area light source may reduce the losses attributed to the backlight units. In order to eliminate color filter loss, a few researchers have disclosed using color organic electroluminescent (OEL) devices or color field emission devices as colored backlight units. One approach (see IBM Technical Disclosure Bulletin, vol. 35, p. 433 1992) proposed using screening techniques to lay down strips of red, green and blue organic electroluminescent (OEL) materials so that the three strips emit individual red, green and blue color. Via sequential switching of the colors, the authors were able to achieve sequential LCD modes. Huang, et al., in U.S. Pat. No. 5,965,907, proposed stacking red, green and blue OEL panels to produce a pixilated color backlight unit. However, the fabrication cost of this unit would tend to be high and color cross talk could possibly occur due to the thick stack configuration. Kumar et al., in U.S. Pat. No. 5,926,239, proposed a colored backlight unit utilizing colored phosphors activated by a field emission device. While the above approaches may reduce color filter loss, they introduce significant difficulties into the manufacturing process. Further, they do nothing to eliminate polarizer losses.
One approach for minimizing polarizer losses is to utilize polarization converters based on integrated polarizing beam splitters (see U.S. Pat. Nos. 5,096,520 and 5,394,253). An advantage of polarizing beam splitters is that they may be used with any type of light source, including those used in conventional LCD backlight units or OEL devices. However, such polarizing beam splitter sheets tend to be bulky and heavy, resulting in application difficulties for most LCD configurations. An alternative approach is to use multiple polymer dielectric layer (MPDL) based polarizers. MPDL based polarizers are constructed of multiple birefringent layers and are designed to reflect the desired polarization and transmit the remainder. Both Benson, in U.S. Pat. No. 5,831,375, and Wortman et al., in U.S. Pat. No. 6,101,032, disclose polarized light sources that utilize MPDL polarizing films. U.S. Pat. Nos. 5,831,375 and 6,101,032 are fully incorporated herein by reference. While a polarized light backlight unit for an LCD may conceivably be made using this technology, it is expected that manufacturing difficulties would be encountered because precise control of the thickness and birefringence value is required for each layer. The necessity of producing a pixilated color polarizer with high spatial resolution would only exacerbate potential manufacturing difficulties.
It is clear that upon review of the prior art, there is no current technology for producing an efficient, colored, polarized backlight unit for an LCD.